title: Regulation of PaCO2 and ventilation in humans inspiring low levels of CO2.
abstract: This study was designed to determine whether 1) arterial PCO2 (PaCO2) increases when inspired PCO2 (PICO2) is increased from less than 0.4 Torr (eupnea) to 7 or 14 Torr, and 2) ventilatory sensitivity to CO2 (delta VE/ delta PaCO2) is greater at low levels of PICO2 (7-21 Torr) than it is at higher levels (28-42 Torr). Human subjects were studied while seated in an environmental chamber that permitted alteration of PICO2 by changing the chamber PCO2. In study 1, arterial blood was sampled over the final 5 min of a eupneic period and again 10-15 min later when PICO2 was 7 or 14 Torr. With this protocol, PACO2 was increased above eupnea by 0.7 (P less than 0.02) and 0.9 Torr (P less than 0.01) when PICO2 was 7 and 14 Torr, respectively. In study 2, arterial blood was sampled every 5 min during two 1-h periods of eupnea that were separated by 3 h during which PICO2 was increased by 7 Torr each 0.5 h. With this protocol there was no consistent difference in PACO2 between eupneic periods and periods when PICO2 was 7-14 Torr. There was a progressively increased hypercapnia as PICO2 was increased from 7 to 42 Torr. The delta VE/ delta PaCO2 was less than half for data obtained at low relative to high PICO2. The two studies demonstrated that measurement error and physiologic variation necessitate using a "powerful" experimental design (study 1) to detect small increases in PaCO2. On the basis of these results, we have concluded that there is no apparent reason to postulate a sensory mechanism other than the carotid and intracranial chemoreceptors to account for the hyperpnea during CO2 inhalation. Specifically, isocapnic hyperpnea probably does not occur.
mesh_terms: D000328:Adult; D001158:Arteries; D001639:Bicarbonates / Q000097:blood; D002245:Carbon Dioxide / Q000008:administration & dosage / Q000494:pharmacology*; D006801:Humans; D006863:Hydrogen-Ion Concentration; D008297:Male; D010313:Partial Pressure; D012119:Respiration / Q000187:drug effects*; D013696:Temperature
chemical_list: D001639:Bicarbonates; D002245:Carbon Dioxide